The electronic protection devices (hereinafter referred to briefly as relays) of automatic circuit-breakers (hereinafter referred to briefly as circuit-breakers) enable detection, via purposely designed sensors, of the conditions of operation, and generate tripping commands which are to cause tripping of the circuit-breaker itself in the case of failures or overloads in the relevant portion of electrical network. By “tripping” is meant in the art the immediate opening of the main contacts of the circuit-breaker. Tripping generally occurs by means of a trip solenoid controlled by the relay.
There exist in the state of the art electronic relays that, in addition to fulfilling the functions of detection and command recalled above, can also provide information of various nature. The availability of said information, which is useful, for example, for the diagnosis of failures and faults, can vary substantially from model to model on the basis of the characteristics of design, and depends in particular upon the main microcontroller and the software installed. In electronic relays of a simple type, there is generally available information limited to the type of failure that has caused tripping of the relay. Said information is then converted via simple interfaces into warnings for the user (for example flags with the warning “short-circuit” or “overload”). More advanced relays, are normally able to manage sophisticated information, which can be translated into more complete and significant warnings. Said warnings can regard, for example, the network frequency, the amplitude of the circulating currents or of the voltages of the phases (supplied as real-time data, or statistical data, recorded for example when a failure occurs), or else the progressive number of the failure, fault, or tripping, or else the interrupted earth current or the interrupted power. Said warnings can also derive from processing, which are in any case complex, of all the information available (for example, statistical data on previous failures, estimation of the residual service life of the circuit-breaker, simulation of the so-called thermal memory). Relays of this latter type render the various information available via warning interfaces of various nature, such as, for example, light or acoustic warning devices, LEDs, or preferentially alphanumeric or graphic displays and digital communication ports.
In electronic relays of a simple type, there are normally used warning interfaces of a bistable type, such as, for example, magnetic warning flags (typically, “short-circuit” or “overload” flags). Said interfaces present the advantage of retaining the indication of the type of failure that has occurred also in the event of absence of the supply, but the information provided by them is somewhat limited.
The most advanced electronic relays are instead based upon the use of software and more sophisticated warning interfaces. These solutions absorb energy, and for their operation the relay requires an electrical supply. Said supply is generally derived with appropriate technical solutions from the same electrical network as the one on which the circuit-breaker is installed (direct supply), or else is derived from external sources (auxiliary supply).
In the case of dropping of the direct supply of the relay (a possibility that arises, for example, after tripping due to failure or fault, or simply in the event of a black-out) and of absence of an auxiliary supply, the relay itself, in order to be able to function at least partially, for example to guarantee the warnings, requires back-up supply systems.
Since the availability of a warning is particularly important precisely after an event of failure or fault, i.e., in the absence of supply, various technical solutions have been experimented and applied for rendering accessible or deducible by simulation some significant information. The simplest known solution consists in equipping the relay with magnetic flags similar to the ones used in relays of a simpler type, and already described in the text. In this case it is, however, possible to set only elementary warnings on the type of failure or fault.
Other known solutions, applied on relays of a more advanced type, tend to render available or to simulate a larger amount of information. Said solutions require that the information which it is intended to have available is appropriately retained by the main microcontroller of the relay or transferred in time to special additional memories operatively connected to the same microcontroller during normal operation. In order to function, these relays are equipped with a back-up supply system, which is activated manually by an operator and will enable activation, at least for a brief time, of the parts of the relay for managing the information and the warning interfaces.
In other words, in all the known solutions, in the absence of direct or auxiliary supply, the main microcontroller undergoes at least one temporary interruption of the supply with consequent arrest of the internal clock. The arrest of the clock causes a series of drawbacks that are well known to persons skilled in the sector. The first drawback consists in the loss of the current time, which must be restored. Other drawbacks are linked to the preclusion of the time variable from the functions of calculation used for deriving complex information.
One of the functions that it would be desirable to calculate precisely using the time variable is, for example, the thermal memory. By “thermal memory” is meant the simulation of an advantageous effect typical of thermal circuit-breakers (of a traditional type, i.e., non-electronic ones), which consists in preventing closing of the circuit by the circuit-breaker immediately after tripping due to overload. In fact, after tripping due to overload the portion of electrical network controlled by the circuit-breaker can be at temperatures close to the values that the device can withstand, and an immediate re-closing of the circuit could prove extremely dangerous. Whereas this function is naturally built into relays of a thermal type, it is virtually lost in the relay of an electronic type, which are substantially independent of temperature.
Various solutions have been experimented and used in the known art for simulating the thermal memory, for example with the use of a capacitor. Said solutions exploit the phenomenon of decay of the voltage across a capacitor charged at the moment of tripping of the relay for estimating the time that has elapsed; on said estimated time, there is then estimated the decay of the temperature in the portion of electrical network controlled by the circuit-breaker.
It is altogether evident that this type of simulation cannot yield sufficiently realistic results for various reasons, due, for example, to the tolerances of the capacitor and to the decay of the electrical characteristics typical of the components. The behaviour of the capacitors is also influenced by temperature, and since this phenomenon is far from controllable or foreseeable, it constitutes a further limit. Furthermore, the decay of the charge of the capacitor is a non-linear phenomenon, with consequent limits of precision.
This approximation results in an undesirable behaviour of the circuit-breaker, for example preventing re-closing of the circuit when this operation is technically safe, or enabling it when it is technically dangerous.
It is therefore evident that the known solutions only partially solve the drawbacks described above, but none of them has proven fully satisfactory.